1. Field of the Invention
The present invention relates to a copying apparatus of electrophotographic type, a fixing roller, and a fixing apparatus for fixing toner on a recording material to the recording material in an image forming apparatus such as a printer.
2. Related Background Art
In the past, although various fixing rollers and fixing apparatuses used with image forming apparatuses, such as general copying machines of electrophotographic type have been proposed and put to practical use, due to excellent mold releasing ability, a fixing roller in which resin of fluorine group is used as an offset preventing layer, has mainly been used. A fixing apparatus of so-called heat roller type having a pair of rollers in which at least one of the rollers is heated by a heat source has mainly be used in consideration of thermal efficiency. FIG. 4 shows a fundamental construction of such a fixing apparatus.
That is to say, FIG. 4 is a partial sectional view of a conventional fixing apparatus of heat roller type. In FIG. 4, a fixing roller 3 has a heat generating source such as a halogen heater 3d and the like therein and is constituted by a metal core 3c made of aluminum and the like. A primer layer 3b including PAI (polyamideimide), coated on the metal core, and an offset preventing layer 3a adapted to prevent occurrence of an offset phenomenon which is made of fluororesin such as PTFE and coated on the layer 3b. A pressure roller 2 is disposed below the fixing roller 3 having such a construction and is constituted by a metal core 2b, an elastic surface layer 2a made of rubber, and the like and coated on the metal core.
The fixing roller 3 and the pressure roller 2 are urged against each other and cooperate with each other to form an appropriate press portion (referred to as "nip portion" hereinafter), therebetween. The fixing roller and the pressure roller are rotated by receiving a driving force of a driving means of a main body of the image forming apparatus. By pinching, (sandwiching), and conveying the recording material bearing an unfixed image by the nip portion between the rollers 2, 3, the unfixed image is fixed to the recording material by heat and pressure.
However, in the above-mentioned fixing apparatus of heat roller type, since the recording material is directly contacted with the surface of the fixing roller 3, a part of the toner on the recording material is adhered to the surface layer, (offset preventing layer), 3a of the fixing roller 3, with the result that a so-called offset phenomenon, in which the adhered toner is transferred onto the recording material again by the rotation of the fixing roller 3, cannot be avoided.
In order to prevent occurrence of the offset phenomenon, generally, the surface layer, (offset preventing layer) 3a of the fixing roller 3 is coated by high mold releasing material such as fluororesin as mentioned above to improve the mold releasing ability. However, since such material has high resistance, the surface layer 3a is greatly charged due to frictional charging between the surface layer and the recording material, with the result that the toner on the recording material is adsorbed onto the surface layer 3a of the fixing roller 3, by the electrostatic action, thereby causing a so-called electrostatic offset phenomenon.
To avoid this, for the purpose of preventing occurrence of the electrostatic offset phenomenon, there has been proposed a technique in which fine particles having low resistance such as carbon black or titanium dioxide or whisker single crystal fibers such as potassium titanate, are added to resin material from which a surface layer 3a of a fixing roller 3 is made to reduce the resistance of the surface layer 3a of the fixing roller 3, thereby preventing the occurrence of the electrostatic offset phenomenon by preventing frictional charging of the surface layer 3a (for example, see Japanese Patent Publication No. 58-23626 and Japanese Patent Laid-Open No. 63-192071).
However, in a fixing apparatus having such a fixing roller, even though the offset can be eliminated effectively, since moisture included in the recording material is injected from the fixing nip portion toward a recording material convey inlet as vapor when the recording material enters into the fixing nip portion, an unfixed image on the recording material is scattered, (so-called "toner scattering" phenomenon). To avoid such toner scattering, it is known to apply voltage having same polarity as that of the developer, (toner), to the fixing roller.
However, when there is no high pressure resistance layer in plural layers on a metal core of the fixing roller for example, if voltage of 700 to 800 Volts is applied to the fixing roller, current will flow to the recording material through the surface layer, with the result that effect of application of voltage becomes less since fixing voltage is reduced. Further, since the charges for holding the developer on the recording material are decreased by the charges flowing into the recording material, the "toner scattering" becomes serious.